Distillation techniques have been used to separate olefin/paraffin mixtures such as an ethylene/ethane mixture or propylene/propane mixture. In such distillation techniques, because there is a small difference in boiling point between olefin and paraffin, distillation columns having a large number of distillation trays are used, thus incurring high energy and equipment costs. Although the off-gas from fluidized catalytic cracking (FCC) processes which produce gasoline and propylene by treating the atmospheric residue or vacuum residue generated in crude oil refining processes contains about 20 vol % of ethylene, the concentration of ethylene in the FCC off-gas is low and the FCC off-gas contains various gases. For this reason, ethylene has not yet been recovered from the FCC off-gas in an economic manner, and the FCC off-gas has been used as fuel gas.
In recent years, technology for reducing olefin separation costs by using a process of separating olefins by adsorption has been studied as a substitute for processes of separating olefins by distillation.
Conventional techniques of separating light olefins (ethylene, propylene, butylene, etc.) and paraffins (ethane, propane, butane, etc.) by adsorption are as follows.
U.S. Pat. No. 6,867,166 discloses a technique of separating olefins by a pressure swing adsorption or temperature swing adsorption process using a transition metal ion-supported adsorbent having selectivity for ethylene or propylene.
Also, U.S. Pat. No. 6,293,999 discloses a technique of separating propylene from a propane/propylene mixed gas by a pressure swing adsorption or temperature swing adsorption process using an ALPO-14 adsorbent having a molecular sieve function which selectively adsorbs only propylene. Moreover, U.S. Pat. No. 6,488,741 discloses a technique of separating C2-C4 olefins by a pressure swing adsorption process or a combination of a pressure swing adsorption process with a distillation process using a zeolite adsorbent. In addition, U.S. Pat. No. 6,488,741 discloses a technique of separating propylene from a propane/propylene mixed gas using an 8-member ring adsorbent having a molecular sieve function, such as SAPO.
As described above, the processes for separating ethylene or propylene by adsorption are carried out by gas-phase adsorption rather than liquid-phase adsorption, because ethylene or propylene is not easy to liquefy. Also, the regeneration of adsorbents is performed by pressure swing adsorption or temperature swing adsorption. Elevating and lowering the temperature of an adsorption column in the temperature swing adsorption process requires a lot of time, so that the productivity of the bulk gas separation process is low, and thus the equipment cost is high. The pressure swing adsorption or vacuum swing adsorption process is not suitable for separating large amounts of mixed gases, because compressors and vacuum pumps have limited capacities.
Korean Patent Registration No. 0849987 registered in the name of the applicant discloses an adsorption/separation process capable of separating ethylene from an FCC off-gas containing a low concentration of ethylene using an ethylene-selective adsorbent. The disclosed process is a displacement desorption process of desorbing adsorbed ethylene using a desorbent and is a technique of concentrating and recovering ethylene from the FCC off-gas through the sequential steps of adsorption, ethylene rinse and displacement desorption. Also, Korean Patent Registration No. 0822847 discloses a displacement desorption process for separating a C4 olefin from paraffin, which comprises an adsorption step, a C4 olefin rinse step and a desorption step.
The above-described olefin/paraffin displacement desorption process is based on adsorption-olefin rinse-desorption steps. In this process, the amount of olefin which is discharged out of the adsorption column in a rinse step is increased depending on the concentration of olefin in the raw material, the adsorption properties of olefin and paraffin, and the required concentration of olefin in products, thus reducing the recovery rate of olefin.
The present invention provides a displacement desorption process that increases both the recovery of olefin and the economic efficiency of the process by recovering olefin which is discharged in the adsorption and rinse steps.